The present invention relates to tunable light sources in the infrared and, more particularly, to a method and apparatus for generating continuously tunable, pulsed, narrow bandwidth light using a periodically poled lithium niobate (PPLN) optical parametric amplifier seeded with the output of a PPLN optical parametric generator.
Many applications such as remote sensing and molecular spectroscopy require broadly tunable sources in the infrared. Some of these applications have the additional constraint of needing narrow spectral bandwidth operation over the tuning range. Achieving broad tunability, narrow spectral bandwidth, and complete spectral coverage within the tuning range has been a difficult task. Systems that meet these requirements are typically complicated and have stringent operating conditions. For example, a tunable optical parametric oscillator (OPO) can cover a large wavelength range and can have a narrow spectral bandwidth at one wavelength, but it is difficult to tune this narrow spectral bandwidth effectively over a broad range.
Transform-limited picosecond pulses have been generated utilizing optical parametric generators (OPG). However, even when the output of an OPG is transform limited, its bandwidth is relatively large, limiting possible applications. For example, high resolution molecular spectroscopy at atmospheric pressures requires bandwidths of less than 0.1 cmxe2x88x921 which is orders of magnitude narrower than the transform limit of a 1 psec pulse of roughly 15 cmxe2x88x921. 
Diode laser seeding of an OPG has been demonstrated as a means for generating a narrow spectral bandwidth. However, although diode laser seeding generally works well, the tuning range of such a system is limited by the tuning range of the diode laser. In addition, these types of systems require two lasers, adding cost and complexity to the overall design.
Accordingly, there is a need for a versatile and cost effective system for generating a narrow bandwidth light source that may be tuned over a broad spectral range while maintaining the narrow bandwidth over the broad tuning range.
This need is met by the present invention wherein a scheme is provided for generating a continuously tunable, pulsed, narrow bandwidth light source using an optical parametric amplifier seeded with the output of an optical parametric generator.
In accordance with one embodiment of the present invention, a method of operating a light source is provided. The light source includes a pump laser, first and second non-linear optical crystals, a tunable filter, and light pulse directing optics. The method comprises the steps of operating the pump laser to generate a pulsed pump beam characterized by a nanosecond pulse duration and arranging the light pulse directing optics so as to (i) split the pulsed pump beam into primary and secondary pump beams; (ii) direct the primary pump beam through an input face of the first non-linear optical crystal such that a primary output beam exits from an output face of the first non-linear optical crystal; (iii) direct the primary output beam through the tunable filter to generate a sculpted seed beam; and direct the sculpted seed beam and the secondary pump beam through an input face of the second non-linear optical crystal such that a secondary output beam, preferably characterized by at least one spectral bandwidth on the order of about 0.1 cmxe2x88x921 and below, exits from an output face of the second non-linear optical crystal. The sculpted seed beam may be generated such that a plurality of distinct spectral bandwidths on the order of about 0.1 cmxe2x88x921 and below exit from an output face of the second non-linear optical crystal.
The pump laser is preferably operated such that the pulse duration is between about 1 ns and about 20 ns. The pump laser may be operated such that the pulse duration is about 15 ns. The light pulse directing optics are preferably arranged and the pump laser is preferably operated to generate a primary pump beam pump energy above a parametric threshold of the first non-linear optical crystal. The light pulse directing optics may be arranged and said pump laser may be operated to generate a primary pump beam pump energy selected to be at least twice the magnitude of the parametric threshold of said first non-linear optical crystal.
The pump laser and the first non-linear optical crystal are typically arranged such that the primary output beam comprises signal and idler components. The light pulse directing optics may be arranged so as to separate the signal and idler components of the primary output beam and direct one of the components away from the first non-linear optical crystal. The method preferably further comprises the step of arranging the light pulse directing optics so as to separate the signal and idler components of the secondary output beam and optically dump one of the components. Preferably, the light pulse directing optics are arranged such that a spot size of the sculpted seed beam on an input face of said second non-linear optical crystal is greater than a spot size of the secondary pump beam on the input face and such that the secondary pump beam spot is contained within the sculpted seed beam spot.
The tunable filter preferably comprises an etalon filter with a characteristic adjustable mirror spacing and the method preferably further comprises the step of adjusting the tunable filter by adjusting the mirror spacing. The method may further comprise the step of continuously tuning the secondary output beam over a predetermined frequency range by scanning the adjustable mirror spacing of the etalon filter. The etalon filter is preferably arranged such that the spectral bandwidth is maintained substantially constant as the output beam is tuned over the predetermined frequency range.
The light pulse directing optics are preferably arranged such that the first and second non-linear optical crystals are separated by an optical path length selected to ensure that an initial pulse of the sculpted seed beam and an initial pulse of the secondary pump beam arrive at the second non-linear optical crystal at substantially the same time. The method preferably further comprises the step of selecting respective operating temperatures and periodicities of the first and second non-linear optical crystals such that respective gain bandwidths of the first and second non-linear optical crystals overlap, whereby the sculpted seed beam falls within the gain bandwidth of the second non-linear optical crystal.
In accordance with another embodiment of the present invention, a method is provided for operating a light source. The light source includes a pump laser, a non-linear optical crystal, a tunable filter, and light pulse directing optics. The method comprises the steps of operating the pump laser to generate a pulsed pump beam characterized by a nanosecond pulse duration and arranging the light pulse directing optics so as to: (i) split the pulsed pump beam into primary and secondary pump beams; (ii) direct the primary pump beam into the non-linear optical crystal such that a primary output beam exits from the non-linear optical crystal; (iii) direct the primary output beam through the tunable filter to generate a sculpted seed beam; and (iv) direct the sculpted seed beam and the secondary pump beam through the non-linear optical crystal such that a secondary output beam, preferably characterized by at least one spectral bandwidth on the order of about 0.1 cmxe2x88x921 and below, exits from the non-linear optical crystal. Preferably, the light pulse directing optics are arranged such that the primary pump beam is directed into the non-linear optical crystal in a first direction and the sculpted seed beam and the secondary pump beam are directed through the non-linear optical crystal in a second direction opposite the first direction.
In accordance with yet another embodiment of the present invention, a method of operating a light source is provided. The light source includes a pump laser, a broadband light source, a non-linear optical crystal, a tunable filter, and light pulse directing optics. The method comprises the steps of: (i) operating the pump laser to generate a pulsed pump beam characterized by a nanosecond pulse duration; (ii) operating the broadband light source to generate a primary output beam; and (iii) arranging the light pulse directing optics so as to direct the primary output beam through the tunable filter to generate a sculpted seed beam, and direct the sculpted seed beam and the pump beam through an input face of the second non-linear optical crystal such that a secondary output beam, preferably characterized by at least one spectral bandwidth on the order of about 0.1 cmxe2x88x921 and below, exits from an output face of the second non-linear optical crystal.
In accordance with yet another embodiment of the present invention, a tunable light source is provided including a pump laser, first and second non-linear optical crystals, a tunable filter, and light pulse directing optics. The pump laser is arranged to generate a pulsed pump beam characterized by a nanosecond pulse duration. The light pulse directing optics are arranged to (i) split the pulsed pump beam into primary and secondary pump beams; (ii) direct the primary pump beam through an input face of the first non-linear optical crystal such that a primary output beam exits from an output face of the first non-linear optical crystal; (iii) direct the primary output beam through the tunable filter to generate a sculpted seed beam; and (iv) direct the sculpted seed beam and the secondary pump beam through an input face of the second non-linear optical crystal. The pump laser, the first and second non-linear optical crystals, and the tunable filter are arranged such that a secondary output beam, preferably characterized a by at least one spectral bandwidth on the order of about 0.1 cmxe2x88x921 and below, exits from an output face of the second non-linear optical crystal.
The first non-linear optical crystal preferably comprises an optical parametric generator. The second non-linear optical crystal preferably comprises an optical parametric amplifier. The optical parametric amplifier may further comprise a wedge arranged on the output face to minimize spurious oscillations.
The light pulse directing optics preferably include (i) a first dichroic mirror positioned to separate signal and idler components of the primary output beam and direct one of the components away from the first non-linear optical crystal; and (ii) a second dichroic mirror positioned to separate signal and idler components of the secondary output beam.
The tunable filter preferably comprises an etalon filter with a characteristic controllable mirror spacing. The etalon filter may include piezoelectric elements arranged to adjust the mirror spacing. The tunable filter may comprise a plurality of etalon filters with respective characteristic controllable mirror spacings.
The light pulse directing optics are preferably arranged such that the first and second non-linear optical crystals are separated by an optical path length selected to ensure that an initial pulse of the sculpted seed beam and an initial pulse of the secondary pump beam arrive at the second non-linear optical crystal at substantially the same time.
In accordance with yet another embodiment of the present invention, a tunable light source is provided including a pump laser, a non-linear optical crystal, a tunable filter, and light pulse directing optics. The pump laser is arranged to generate a pulsed pump beam characterized by a nanosecond pulse duration. The light pulse directing optics are arranged to (i) split the pulsed pump beam into primary and secondary pump beams; (ii) direct the primary pump beam into the non-linear optical crystal such that a primary output beam exits from the non-linear optical crystal; (iii) direct the primary output beam through the tunable filter to generate a sculpted seed beam; and (iv) direct the sculpted seed beam and the secondary pump beam through the non-linear optical crystal. The pump laser, the non-linear optical crystal, and the tunable filter are arranged such that a secondary output beam, preferably characterized by at least one spectral bandwidth on the order of about 0.1 cmxe2x88x921 and below, exits from the non-linear optical crystal. The light pulse directing optics are preferably arranged to direct the primary pump beam into the non-linear optical crystal in a first direction and direct the sculpted seed beam and the secondary pump beam through the non-linear optical crystal in a second direction opposite the first direction.
Accordingly, it is an object of the present invention to generate continuously tunable, pulsed, narrow bandwidth light in the infrared using a versatile and cost effective light source. A further object of the invention is to provide a two-crystal arrangement that allows for narrow spectral bandwidths, broad tunability, and high conversion efficiency using a single nanosecond pump source. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.